This invention relates generally to electrooptic display (EOD) systems and display systems wherein the characters are formed by action of a light or laser beam.
The prior art in liquid crystal displays (LCD's) has been segment- or matrix-addressed, using a wide variety of line, multiplexing and drive circuit techniques. A few light energy addressed LCD's have been reported and are of two general types: image addressed and laser beam addressed displays. In both of these type displays, the liquid crystal (LC) cell or panel is not viewed directly but is optically projected onto a screen. The LC cell and projection optics can be either a reflective type or transmissive type. In the reflective types, a light source in the form of a desired image is focused onto the rear side of an LC cell where the image falls on a photoconductor coating and an absorbing layer which prevents any light from passing from the rear to front of the cell. The LC material in that portion of the cell receiving the image changes from a clear state to a scattered, and therefore, opaque state. A reflective coating is placed behind the LC material, so that projection light from a source in front of the cell is reflected forward to projection optics and to a display screen. The transmissive type of LCD has an LC cell without absorbing or reflecting layers, so that writing light entering from the rear is passed through the cell. The scattered regions of the LC material blocks the light and the image appears black when projected to a screen.
Each of the above prior arts suffer from several difficulties which, to date, have prevented them from becoming a practical display device. The matrix addressed displays are very difficult to build in large sizes (i.e., larger than 4 inches square), and they are restricted to low display resolution, since the resolution is controlled by the number of X and Y lines per unit area and the necessity of connecting each of the lines to a drive circuit. The image addressed LCD's reported have the disadvantage of requiring an expensive cathode ray tube (CRT) to provide an image to the LC cell. The laser addressed LCD's reported have the following disadvantages: (1) they require high intensity light source and optics to project the LCD image to a screen; (2) their LC cells are small requiring high magnification; (3) the space taken up by the display apparatus is large because of the long optical path required; (4) light loss occurs during projection and results in low contrast; and (5) their displays tend to wash-out in a high ambient light environment.
The invention claimed here has the inherent advantages of being a direct view (i.e., no projection optics), low profile display device having an optimized viewing angle. The display panel can be arranged to have a viewing angle less that 90.degree. from horizontal which is an improvement with respect to human factors over conventional display monitors. Since the display panel is inclined, the overall height of the display system and enclosure is reduced. The compactness of the device lends itself to portability. The display device described by this invention could be a suitcase portable display monitor or terminal. Another advantage of this invention is that the display panel is flat, even for large sizes, unlike CRT's which are curved. Finally, the low profile, inclined display panel is ideal for use with transparent graphic stylus or pen tablets or touch panels (i.e., light pen mode).
The prior art with respect to LC display cell or panel construction shows us methods used to build a liquid crystal electrode sandwich. A typical LC cell is comprised of a first glass sheet coated with an indium tin oxide layer for the bottom electrode, a thin 1/2 mil MYLAR or TELDAR spacer around the perimeter of the viewing area, a thin liquid crystal compound layer of 10-15 microns thick, and a second glass sheet also with an indium tin oxide coating. The perimeter is typically sealed with a glass frit. Prior to sealing and liquid crystal injection, the surfaces of one or both of the electrodes are usually cleaned and treated to facilitate the long rod-like liquid crystal molecules in aligning either parallel or normal to the cell walls. The prior art covers several surface alignment techniques which is well known to those skilled in the art. All liquid crystal cells or panels of the prior art are relatively small in viewing area because of the difficulty in maintaining the thin 10-15 micron gap between the glass sheets over the entire cell area. Most cells are less than 2 inches square. The largest prior art liquid crystal panel reported is a circular 6 inch diameter cell. However, the realization of that cell was only possible after a very costly and time consuming fabrication process and is not appropriate for general purpose display device applications. The large area liquid crystal panel of this invention is capable of being manufactured in large quantities on a production basis. The method of this invention enables the fabrication of panels up to a size of 121 square inches or larger, depending on manufacturing techniques and physical constraints. In addition, these large panels are capable of being viewed directly by an individual with or without image projection. The advantages of direct view over projection have been stated above; however, even if a projection method is used for a particular application, the larger LC panel will mean a lower magnification, a shorter optical path, and a smaller device enclosure.
Prior art in the field of character generation for laser beam addressed liquid crystal displays have been reported by (1) D. Maydan, et al, ("IEEE Conference Record" of 1972 Conference on Display Devices) with combination of a TeO.sub.2 acoustic-optic deflector and a galvometer scanner; and (2) A. B. Dewey, et al, (Proceedings of the SID, Vol. 19, No. 1, 1st Quarter, 1978) with a pair of galvo scanners. In each technique, alphanumeric characters are formed by a raster scan method using a modulated laser beam to form the desired characters. These methods have several disadvantages. First, the characters are formed by a dot pattern; thus the characters are not distinct or clearly formed, especially if the character dot matrix is less than 7.times.9. If the matrix size is increased, readability is improved, but the character write rate is reduced because more scans are required per character. The write rates using this method on large panels are relatively slow. Finally, the scanners and/or deflectors required to scan the small rasters, must be highly accurate and fast (high frequency response), which means they are very costly. The laser beam shaping aperture character generator of this invention has the following advantages. The alphanumeric characters formed by the laser beam will be much sharper and more well defined than dot matrix characters which greatly improves the readability of the text displayed on the screen. Secondly, the aperture plate or template can be removable, providing means for user-changeable-character sets for displaying characters of different languages or displaying special characters. Another advantage of this character generator when used with EOD's is that it provides faster character write rates than the dot matrix technique. This is especially desirable in LCD's because of the relatively slow response of the liquid crystal material. Finally, the aperture plate provides a simple means of producing a plurality of beam diameters which can be used to produce several different line widths on the display panel in order to convey information emphasis and graphics.
The prior art involved with laser scanners show us a diversity of electro-mechanical, acousto-optical, electrooptic, and refraction defices and techniques. Only electro-mechanical devices show any promise of providing large angle (greater than +/-30.degree.) deflection required in direct view displays. There are several electro-mechanical scanners available: galvonometer-mirror, piezo-electric fiber, torsion bar, and rotating polygon devices. The best of the prior art scanners, which satisfies the normal display requirements of large angle deflection, analog responses, good linearity, broad band frequency response, and non-periodic wave pattern capability, is the galvo-scanner. For X and Y deflection, two galvo-mirror scanners, a 90.degree. mirror, and a mounting frame are usually required. The magnetic fiber optic laser beam scanner of this invention has several important advantages over the galvo-mirror and other prior art scanners. The fiber optic scanner is an inherently simpler device having only one moving part as opposed to two for the galvo-mirror scanner. The less complicated construction will result in a lower cost and a more reliable device. The fiber optic element can be made very small, so that the mechanical response and dynamic performance (lower inertia) is better than the galvo device, resulting in a frequency response that is moderate to high. Another advantage of the fiber optic scanner is the capability of wider angle deflections of greater than .+-.60.degree.. The overall size of the device is smaller in both mass and volume, which is a great advantage in many applications. Finally, an important advantage over the galvo-mirror scanners is the elimination of mirror alignment problems which effects the mechanical design.
Various absorbing layers or photoconductor coatings have been reported in the field of image addressed LC cells, all of which were intended for reflective types LC light valves. Although they absorb most of the laser energy, 65%-98% depending on the type, some of the laser energy still passes through the cell. E. A. Cunningham, et al., (IBM Technical Disclosure Bulletins, Vol. 18, No. 11, April 1976) reports three types of absorbers: (1) antireflection coatings ZnS and MqF; (2) a thin layer of germamium; (3) a layer of polymer doped with a dye matched to the laser. T. D. Beard, et al., (Applied Physics Letters, Vol. 22, No. 3, Feb. 1, 1973) uses CdS photoconductor with a CdTe layer to block any residue light getting through to the photoconductor. When a laser beam is applied to an absorbing layer, a localized heating takes place. It has been shown that when an absorbing layer of photoconductor coating is placed at or near an LC material, the molecular phase of the LC material is locally changed by the application of the light source. There are several advantages of the opaque absorbing layer of this invention when the layer is placed between the LC material and the rear transparent electrode. The most important advantage is that this layer will block 100% of the energy from the laser beam and thereby eliminate any laser safety problems. Because of the two states of the LC material, either milky white (scattered molecules) or transparent (aligned molecules), a black-on-white background or white-on-black background display is possible by making the color of the opaque absorbing layer black. With the black-on-white display, which is the preferred embodiment, normal ambient room light would be sufficient to produce a high-contrast display. Also, a black-on-white display would reduce the annoyance of reflective glare off of the display screen. This is especially important if the screen angle is inclined because it will tend to reflect ceiling lights. Finally, this layer is adaptable to being edge-lighted around the perimeter such that the scattered LC molecules will propagate the light over the entire display background.
Other attributes and advantages inherent in the system herein disclosed will be readily apparent to one of ordinary skill in the art.